Analysis systems, such as mass spectrometry (MS) systems, optical emission spectrometry (OES) systems and the like, can be used to analyze the composition of a target material, for example a solid crystal. Often, a sample of the target is provided to analysis systems of this type in the form of an aerosol. The aerosol is typically produced by arranging the target material in a sample chamber, introducing a flow of a carrier gas within the sample chamber, and ejecting a portion of the target in the form of particles. The ejecting may be done for example by laser ablating a portion of the target with one or more laser pulses, from a laser, to generate a plume containing particles and/or vapor ejected or otherwise generated from the target suspended within the carrier gas. Thereafter, the ejected particles are typically entrained by the flowing carrier gas and transported to an analysis system via a sample transport conduit. These analysis systems perform applications including Laser ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) and Laser ablation Inductively Coupled Plasma Optical Emission Spectrometry (LA-ICP-OES)
Analytical laser ablation applications require repeated movement of an XY, or XYZ, positioning system during experimental runs performed subsequently to a scan placement process in which movements to be repeated during the experimental run are set. One exemplary use of analytical laser ablation systems is for zircon crystal grain dating. Zircon crystals incorporate uranium and thorium atoms into the crystalline structure, and strongly reject lead during the formation of the crystal. Therefore, any lead present in a zircon crystal is assumed to be from radioactive decay. Therefore, if the composition of a zircon crystal is determined the age of the crystal can be determined by calculating the amount of time it would take to produce the ratio of uranium to lead in the crystal through radioactive decay.
Zircon crystal grains used for dating frequently have very small dimensions, for example a single grain may range in size from 20 um across to 200 um across points in an outline of the crystal grain. Within the crystals invisible structures may exist that can be imaged using an SEM, XRF or other similar tool. These invisible structures may cause areas of a crystal grain to be not ideal for certain analytical laser ablation applications. For these reasons a desired target on a zircon crystal grain for zircon crystal dating, and similar applications, may be a very small and thus these applications require very high precision for ablating a surface of a crystal grain, and therefore require high precision stage movement.
During a typical zircon crystal grain aging application an exemplary sample slide may be prepared containing approximately 20-200+ grains in an approximately 50 mm by 50 mm area. The grains may be placed on the slide automatically or by a user. The grains may be placed in a random arrangement or in an orderly pattern, such as in rows and columns. Further, the grains may be placed according to sets of crystal grains to be analyzed together. After placement the grains may be machined to have flat surfaces substantially on the same plane as the other grains on the sample slide.
After the sample slide is prepared it is loaded into a sample chamber of an analytical laser ablation device and a scan placement process is performed. An operator places a pattern scan, also referred to as an overlay, on the sample slide. This may be done with using software to perform a virtual overlay of pattern shapes. During this process the locations of a series of scans or holes to be fired upon by the laser are set in precise positions, for example at specific locations on the machined faces of individual crystal grains. These set positions are referred to as intended laser locations. Once the scan placement process is complete an experimental run occurs where a motion control system executes a series of movements determined by the scan placement process to ablate each crystal grain at the intended laser locations at a desired time and sequence.
As part of the experimental setup a reference material blank is usually placed in the sample chamber of a laser ablation apparatus, such as off to one side of the main experiment area. The reference blank has a known composition. The system may be set to analyze sets of zircon crystals grains and between sets of zircon crystal grains the system will then be programmed to sample the reference blank material. In this way, analytical drifts measured at an ICP-MS for example can be corrected for given the reference's known and repeatable concentrations of material.
During the scan placement process a list of intended laser locations are saved as XY, or XYZ, stage coordinates. During the experimental run, for each intended laser location the sample is moved relative to the laser by the motion control system according to the saved coordinate position.
During an experimental run a motion control system will move the sample slide relative to the laser for each incrementally setup intended laser location on a zircon crystal grain in a set in the sample and then to the reference blank, then back to the next set of crystal grains. With each large movement either between crystal grains with intended laser locations, or to the reference blank a bi-directional repeatability error may appear to shift the sample slide relative to the laser beam's position. A large 30 mm move can incur a built up bi-directional repeatabilty or accuracy error that will shift the laser focus position off of the intended laser position on a crystal grain. The precision with which the ablation pattern was placed relative to the sample will be reduced by the time the laser is to be fired if a repeatability error accumulates. In some cases the laser will fire in an unintended location, including missing the crystal grain with an intended laser location thereon altogether. This is undesirable since a laser firing at an unintended location will skew or ruin the data for that experiment pass.
Due to the large number of zircon crystals often sampled during the same experimental run, such as 20 to 200+ crystal grains, it is undesirably time consuming for an operator to monitor the equipment and correct for poor system level accuracy of the laser beam on the sample during the experimental run.
Conventional techniques for XYZ positioning systems include motion control topologies, such as open loop and closed loop. Open loop designs may be stepper motor based, and move the stage via linear type motors or lead-screw drive type mechanisms a precise amount, for example a fraction of the actual full step range. Each step can be on the order of 1-2 um of XY movement, with microstepping adding a divide by 2, 4, 8, 16 or 32 microsteps per full step. In this way it is possible to attempt positioning at a very high resolution, only limited by the mechanical coupling of the stage mechanism. Closed loop adds to this a feedback mechanism and a controller topology that attempts to reduce requested position-actual position error until the error is zero or very small. These solutions have the disadvantage of requiring costlier stages and controllers and are complicated to implement.